Missile guidance laws for a strapdown seeker with a narrow field-of-view

Abstract

New guidance laws are proposed to solve the problem of when a missile is equipped with a strapdown seeker instead of a gimbaled seeker. The strapdown seeker has advantages of relatively simple implementation compared to a gimbaled seeker, and it can eliminate frictional cross-coupling significantly save on costs. There have been many studies to enable guided missiles to use strapdown seekers, but they have several weaknesses, such as measurement error caused by scale factor error, radome errors, glint noise, narrow field-of-view (FOV), and so on. Among these weak points, focus is centered on the narrow FOV of the strapdown seeker. A hybrid guidance (HG) law is proposed to maintain the lock-on condition in spite of the narrow FOV of the strapdown seeker. The proposed HG law consists of two guidance phases, which assume operation at a switching boundary. In the first phase, the proportional navigation guidance (PNG) law is applied during the time when the look angle is inside the switching boundary. For the second phase, when the look angle is outside the switching boundary, a new guidance law is derived to keep the look angle within the FOV by employing a Lyapunov-like function based on sliding-mode control methodology. The appropriate determination of the switching boundary is an important issue. The idea behind selecting the switching boundary is to use the PNG law as much as possible, and to make the missile stay in the lock-on condition. A lock-on guidance (LOG) is proposed as another approach to solve the problem of narrow FOV, based on the concept of the pursuit guidance (PG) law. In order to derive the LOG law, we use a Lyapunov-like function based on the sliding-mode control methodology. An advantage of the LOG law is that a missile guided by the LOG law can intercept a target with a very narrow FOV of the strapdown seeker. Because such a seeker often has to be implemented for more accurate measurements, this kind of guidance law is needed to prepare for such a situation. The LOG law is simple and has good performance against a target with high speed.